1. Field of the Invention
This invention relates generally to an adjustable shock absorber for motorized vehicles and, more particularly, to a shock absorber having a plurality of adjustable valves disposed in the flow path of a damping fluid or other working medium according to which adjustments may be made to the damping curves and break over characteristics of the shock absorber while the shock absorber is installed in a vehicle.
2. Description of the Prior Art
It is known in the art of vehicle suspension design that shock absorbers may be placed between relatively movable portions of a vehicle suspension, such as between the frame of the vehicle and the struts supporting each wheel, in order to reduce the forces transmitted from the wheels to the automotive frame and to damp out unwanted oscillations caused by the vehicle mass acting against a suspension spring or other resilient member disposed in the vehicle suspension.
Typically, such shock absorbers are comprised of a piston and cylinder arrangement in which a damping fluid or other working medium is caused to pass through a defined aperture or orifice. The flow of working medium through the orifice produces a characteristic damping curve according to known principles of fluid dynamics. For an incompressible viscous fluid, for instance, the Bernoulli equation predicts that the damping force will be proportional to the square of the flow velocity through the orifice. See, eg., Robert W. Fox, Alan T. McDonald, Introduction to Fluid Dynamics, 3rd Ed. 1985, pp 391-395.
It is also known to vary the damping characteristics of a shock absorber, as desired, in order to accommodate for changes in the mass of the vehicle and its load, stiffness of the suspension springs, and the particular characteristics of the shock forces imparted on the wheels as the vehicle travels over a road or off-road surface. For high frequency shock forces, for instance, corresponding to high speed travel over a bumpy road, it may be desirable to decrease the damping function of the shock absorber in order to reduce the damping forces exerted on the vehicle frame. In other driving conditions it may be desirable to increase the damping function of the shock absorber in order to reduce the peak overshoot for low frequency inputs and to provide a faster settling time.
For competition motorcycle suspensions, it is particularly important that the shock absorbers be "tuned" to the type of terrain over which the motorcycle will travel. Unlike conventional automotive suspensions, which are designed mainly for use on paved roads, motorcycle suspensions must accommodate a variety of terrain types present in off-road travel conditions. Similarly, in high-speed automotive racing it is advantageous to be able to "tune" the suspension system to the particular track on which the car will be traveling. A tighter or looser suspension may be desired, depending upon the particular type of pavement surface, shape of the track, and presence of imperfections or debris on the road surface. In each instance, it is desirable to adjust the shock absorber to provide optimal performance for the particular road or terrain conditions present.
One way to accomplish this adjustment function is to provide a shock absorber with removable inserts defining discrete orifices corresponding to known damping curves. By changing the size of the orifice, one can vary the damping characteristics of the shock absorber in a predictable manner. A drawback of this type of design, however, is that the shock absorber must be removed from the vehicle and disassembled in order to make each adjustment. This can be a time consuming process, for instance, in cases where trial-and-error adjustments are necessary to fine tune the suspension system of a vehicle before a race. Also, only discrete adjustments may be made. It is not possible, for instance, to provide a continuous range of adjustments due to the finite number of interchangeable inserts that can be made available.
To overcome these drawbacks, it is known to provide a shock absorber with one or more variable orifice valves for adjusting the damping function of the shock absorber. One such system is described in U.S. Pat. No. 2,780,321, issued to C. Sturari of Milan, Italy. That patent discloses a conventional fluid-damped shock absorber having adjustment means disposed at one or both ends adapted to vary the size of metering ports through which a damping fluid passes under the influence of a piston moving within a cylinder. At one end, the adjustment means consists of a plurality of balls seated in a narrow bore leading to the piston rod axial bore. On one side, the balls engage the end of a threaded screw fitted in the shock absorber casing. On the other side, the balls engage the end of a rod fitted in the piston rod axial bore. As the screw is adjusted the balls cause the rod to move axially, allowing for the adjustment of a needle valve or other variable orifice valve disposed within the fluid-contacting surface of the piston. Other adjustable shock absorbers are known which incorporate an electric solenoid for operating a variable orifice disposed within the piston.
The placement of variable orifice valves in a movable piston requires the implementation of fairly sophisticated control elements, such as balls, linkages, extension rods and electric solenoids, for adjusting the variable valve. This control arrangement has certain undesirable characteristics from a manufacturing standpoint as it necessitates providing many additional components for transmitting the adjustment forces to the centrally disposed valve. This increases the expense of manufacturing the adjustable shock absorber.
Furthermore, due to the added complexity of the adjustment mechanisms and the space occupied thereby, it is often necessary to reduce the piston diameter of the shock absorber in order to accommodate the additional components in a single cohesive unit. This creates additional undesirable characteristics in that the shock absorber is then required to operate at relatively high pressures; e.g., pressure levels in excess of 725 lb/in.sup.2. Under these high-pressure conditions, the forces exerted on the working medium cause it to behave like a spring, producing undesirable hysteresis effects on the damping curve that are not easily predicted or controlled.
Adjustable shock absorbers present the additional design problem in that they have a much wider range of operation than conventional shock absorbers. Where it is desirable to operate a shock absorber with a particularly high damping function, for instance, very high pressures may be generated in the shock absorber in the event that a high velocity displacement is encountered. These high pressures could cause the shock absorber to rupture or impart dangerously high forces on other components comprising the vehicle suspension system.